In the production of silicon or silicon alloys in a low-shaft electric arc furnace, it is known, as the discussion below will demonstrate, to utilize raw material bodies, blanks or elements which are composed of quartz sand, a noncaking carbon carrier, preferably petroleum coke, and a pitch-containing binder.
A composition of this type is formed by mixing the quartz sand, the noncaking carbon carrier and the pitch-containing binder and subjecting the mixture to briquetting thereby forming green briquettes or preforms. These briquettes then subjected to a hardening heat treatment to produce the raw material bodies with sufficient structural stability that they will retain integrity over at least part of the silicon or silicon alloy producing process occurring in the low-shaft electric arc furnace.
In the production of such raw material bodies, quartz sand in the particle size or grain range of 0.05 to 0.2 millimeters is usually employed and indeed with various grain-size bands within this range. It will be understood that the raw material bodies for the production of ferrosilicon can also contain finely divided iron or finely divided iron oxide. The iron or the iron oxide can also be added directly to the burden of the low-shaft electric arc furnace, either in the form of pieces or pellets. The burden predominantly consists of the raw material bodies and quartz.
Depending on the specific use to which the raw materials are to be put, they just have certain chemical and physical properties.
Thus for the production of silicon, in chemical terms the raw material bodies must have a composition (see the Great Britain Patent 2 084 122) such that, in their passage through the low-shaft electric arc furnace, in the upper part, they sustain a reaction according to the equation EQU SiO.sub.2 +3c=SiC+2CO
leading toward the formation of silicon carbide. For that purpose, the total carbon content of each individual raw material body must be correspondingly selected. In general, however, a superstoichiometric operation is preferred (see Great Britain Patent 2,150,128). In physical terms the raw material bodies must have sufficient strength and integrity that they will not deteriorate during passage at least through the upper portion of the low-shaft electric arc furnace and especially in the reaction which forms the silicon carbide so that the bodies can reach the lower part of the electric arc furnace where they can react further with the molten quartz in accordance EQU SiO.sub.2 +SiC=3Si+2CO.
It will be understood that in the low-shaft electrode arc furnace further reactions can be carried out. Among these are: EQU SiO.sub.2 +C=SiO+CO
which occurs at high furnace temperatures to generate volatile silicon monoxide which can be lost and can have a detrimental effect on the output of the process and the heat balance.
In an earlier process from which the present invention has developed (see German Open Application 37 24 541), the raw material bodies free from caking coal are formed by a binder briquetting. The hardening heat treatment is effected in a heated rotary kiln or rotary tube furnace which has its lower portion so filled with quartz sand that the heat treatment takes place by immersion of the briquettes in the quartz sand bed. This has been found to be highly satisfactory. The hardened raw material bodies are able to withstand all of the stresses arising in the process during the transiting of the low-shaft electrode arc furnace and hence the chemical processes which are carried out are particularly well defined.
However, the bodies do not have satisfactory long term storage capabilities in many respects and as to such long term storage properties, therefore, improvements are possible. For example, the mechanical strength of such bodies decreases with time to the point that the bodies may deteriorate. It has been found that such deterioration may be a result of diffusion of air and moisture into the bodies when they are stored in the open and that such diffusion may reduce the binding force of the binder which is used. This is not the case with raw material bodies for the same purpose which may be fabricated by hot briquetting techniques (see German Patent Document DE-OS 30 09 808), but the hot briquetting process is relatively more expensive.